Illicit sexual acts within marriage are equivalent to fornication and adultery, as Saint Jerome taught
“And it makes no difference how honorable may be the cause of a man’s insanity. Hence Xystus in his Sentences tells us that ‘He who too ardently loves his own wife is an adulterer.’ It is disgraceful to love another man’s wife at all, or one’s own too much. A wise man ought to love his wife with judgment, not with passion. Let a man govern his voluptuous impulses, and not rush headlong into intercourse. There is nothing blacker than to love a wife as if she were an adulteress.” (St. Jerome, Against Jovinianus, Bk 1, n. 49)
Note that St. Jerome states that “it makes no difference how honorable may be the cause of a man’s insanity.” The intent, he argues, that motivates man to sin is irrelevant, to the morality of the act itself. If a sexual act is a sinful by nature, it does not matter how honorable or dishonorable the man’s intentions are, it is still a serious moral disorder, comparable, as a figure of speech, to the serious mental disorder of insanity.
St. Jerome taught that there are sexual sins within marriage. The idea that “nothing is shameful” as long as the marital act occurs at some point in time is plainly rejected by St. Jerome. It is contrary to wisdom and good judgment for a man to have sexual relations with his wife in an disordered manner. Though St. Jerome does not, like modern-day moral theologians, give explicit descriptions of various sexual acts, it is clear that he rejects the idea that the mere deposit of the right bodily fluid from a missionary position open to creation is all he is supporting.
“Husbands, love your wives, just as Christ also loved the Church and handed himself over for her,” [Ephesians 5:25]
In his commentary on the above verse, St. Jerome says: “In marriage, the acts that make for children are allowed; but the pleasures which are taken from the embraces of prostitutes are condemned in a wife.” This Jerome condemns the nature of the act as evil, the partner is but a by-product of the action.
Above, Jerome condemns sexual acts with a prostitute, for he says that the acts in question are “condemned in a wife”. What acts are these? They are sexual acts ordered toward pleasure without the procreative meaning. In modern terms, these are sexual acts which are inherently non-procreative, making them intrinsically evil acts. However, nothing of this is directly found in our Catholic Catechism for guidance. Except, birth control methods including elected surgery. Once in a confessional, I asked a elderly priest his opinion, and his response was, if a Catholic married couple behind close doors, the church is in essence silent. However, asking another priest about the act of sodomy in marriage, he was repulsed, almost stunned to hear this could occur with a married couple!
I suppose neither priest was aware of Jerome thoughts on the subject. But now you are!
St. Jerome, Latin in full Eusebius Hieronymus, pseudonym Sophronius, (born c. 347, Stridon, Dalmatia—died 419/420, Bethlehem, Palestine; feast day September 30), biblical translator and monastic leader, traditionally regarded as the most learned of the Latin Fathers. He lived for a time as a hermit, became a priest, served as secretary to Pope Damasus I, and about 389 established a monastery at Bethlehem. His numerous biblical, ascetical, monastic, and theological works profoundly influenced the early Middle Ages. He is known particularly for his Latin translation of the Bible, the Vulgate, and is considered a doctor of the church. Source: https://www.britannica.com/biography/Saint-Jerome]